Russell McMahon wrote... >>> Buck converters are usually best run in continuous current mode ... > >> I see nothing inherent to buck converters to support that statement. > >Continuous and discontinuous modes are quite different in the = relationship >between input and output voltages. The continuous mode can be considered= as >a PWM waveform filtered by the output LC filter. The discontinuous mode = adds >extra terms for frequency of switching and inductance in the Vin/Vout >relationship.=20 All this says is that if you want to model the action of these two different operating modes mathematically, you end up with two different sets of equations, one to describe each mode. And if the circuit is allowed to go back and forth from one mode of operation to the other, juggling these two different mathematical models becomes a real pain in the neck. But the circuit itself cares naught about mathematics: it just sits there and dutifully oscillates, turning on and off the flow of current through the inductor to keep the output voltage within its prescribed bounds. >Swapping between modes in one design is generally >ill advised as design criteria for both are hard to meet simultaneously. Why is this an issue? Olin took great care to ensure his design ALWAYS operates in the discontinuous-conduction mode, and simulation results suggest that's precisely what it does. You're aware, aren't you, that your own switching regulator design (as shown in picbful.gif) engages in this allegedly ill-advised practice of "swapping modes"? Yes, that's exactly what it does: run it with a high enough load current and/or a low enough input voltage, and it operates with continuous inductor current. Supply it with a high enough input voltage and/or a light enough load, and it operates in discontinuous current mode. And when it transitions from one mode to the other with a swept load, what happens is... absolutely nothing. No sudden change in output voltage; no transients of any kind; no instability or anything else. It just sits there and does its thing. So what's so "hard" about this? >Arguably the continuous mode is most desirable.=20 Like your earlier assertion that "buck converters are usually best run in continuous current mode", this is simply untrue as a general statement. Sometimes operating a stepdown converter this way is advantageous; sometimes, it's not. It all depends on the performance characteristics you're trying to attain. To make the sweeping generalization that "continuous mode is best" is, quite simply, WRONG. >That said, any buck >converter will be forced to run in discontinuous mode under a light = enough >load and this may well be acceptable for a normally continuous mode = design . > >Texas Instruments report SLVA057, "Understanding Buck Power Stages in >Switchmode Power Supplies" March 1999 gives a good overview of the two >modes and their differences. >They note - > > "It should be noted that the buck power stage is rarely > operated in discontinuous mode in normal situations, > but discontinuous conduction mode will occur anytime > the load current is below the critical level." "Rarely operated in discontinuous mode"? =20 Sometimes you have to do a bit of translation on these application notes. I translate the above to read, "Some of TI's competitors make some very nice switching regulator chips that operate in discontinuous conduction mode. Maxim even has a trademark for their version of that mode, as used in their ultra-low power, high-efficiency switching regulator chips: they call it Idle Mode(tm). We don't want you to buy their chips, we want you to buy ours!" =20 Caveat emptor. >This of course doesn't mean you shouldn't design a discontinuous mode >version if there are major advantages in doing so. When designing switching regulators to operate efficiently at very low power levels, the advantages of discontinuous mode operation are not merely "major": they're downright compelling. Consider the sorry state of affairs we find ourselves in when we try to design a low-current switcher which operates in the continuous conduction mode. As always with inductors, V =3D L * (dI/dT). Since dI (i.e., the change in inductor current in each part of the conduction cycle) has to be small if we're to avoid having the inductor current drop to zero at any point in the cycle, and the supply voltage V is fixed, we have two choices: we can either make L very large, or we can make dT very small. Neither of these choices is very pretty. =46or examples of the latter approach, check out recent offerings from Linear Technology and from Maxim. Some of their newer switching regulators operate at over a megahertz (!), which allows them to use reasonable size inductors. That's fine for them, because they're integrating these things on silicon and can achieve very high speeds; but we peons don't have that option with discrete, home-built stuff. The other approach, increasing L, is REALLY ugly: bigger inductors are physically large (bad), lossy (bad), and expensive (bad). For an example of this approach, check out an old appnote from Linear Tech at http://www.linear-tech.com/pdf/an30.pdf and look at Figure 24 on the bottom of page 17. This is a micropower switching regulator using an LT1017 dual comparator chip, a 74C907 CMOS open-drain buffer chip (there weren't any really decent P-channel power MOSFETs back in those days), and a one hundred millihenry inductor! Ridiculous. And the problem just keeps getting worse, the lower the output current (and, by the way, the higher the input voltage). At low power levels, discontinuous-conduction mode becomes the only way to fly: it allows the use of small inductors (check out Olin's circuit, which uses a 10 microhenry inductor). And small is cheap. And cheap is good. This is, after all, the PICLIST... Dave -- http://www.piclist.com hint: PICList Posts must start with ONE topic: [PIC]:,[SX]:,[AVR]: ->uP ONLY! [EE]:,[OT]: ->Other [BUY]:,[AD]: ->Ads